CMP.12:4.3 - Make control, progress and resource use executable

Decide what remains to be done after the current subexpression returns. A recursive evaluator can retain that continuation in its own call stack. An explicit evaluator can instead store pending operations, environments and return destinations as data. Construct the transition that resumes the pending work with the obtained result.

Distinguish the termination of translation from the termination of the translated program. A compiler that traverses a finite syntax tree can terminate even when the generated program runs indefinitely. A step limit can return a suspended computation; reaching that limit does not establish divergence. CMP.1 supplies the limit on general termination decision when that question arises.

Count consequential dispatch, environment lookup, retained continuations, arithmetic and storage. Removing repeated syntax analysis may repay compilation for repeated execution. For a rarely used or frequently changing expression, direct interpretation may be the cheaper choice. Retain the existing algorithmic cost and comparison methods rather than assuming compilation always improves the work.